A ladder to the Universe: Time and Distance in Space

Usually when we think about finding the distance to an object, we pull out a ruler or tape measure and get to work. But how do we find distances to things we can't see and aren't even on Earth? How can we really know how far away a star is for example? The key to understanding how astronomers calculate the distances to the most distant objects begins by taking the first step on the cosmic distance ladder. The first rung of this ladder is called parallax. To visualize this technique, extend your hand and raise your thumb. Now close one eye and take note on the background surrounding your thumb. Open the eye that was previously closed and close the eye that was open, but keep your hand extended. What has happened to your thumb? Does it appear to be moving between closing and opening one eye? Your thumb isn’t moving but the position you are viewing your thumb is changing. This phenomenon is parallax. We take advantage of this property in order to calculate the distances to stars. For Earth’s case, scientists point their telescope at a star, record its location and six months later record the same star’s position.  The reason they wait six months is because at six months, Earth will be on the opposite side of the Sun. From these measurements, scientist can calculate the angle that the observed star has moved and use that angle and the known distance between the Sun and Earth to calculate the distance to the star.  The diagram below shows how parallax works for Earth. Parallax is limited by the angle that can be measured by scientists. The smaller the angle becomes, the less reliable the measurements become. Additionally, a smaller angle means that the star appears to be ‘moving’ less between measurements, which is the foundation of this method. 

Figure 1 Trigonometry of Parallax

Once the stars stop ‘moving’, it is time to take the next step on the ladder. This is when we start using Standard Candles. Imagine that we have a candle and its directly in front of us. This candle appears to be so bright that you may feel the need to protect your eyes. Now this candle begins to move further and further away from you. As its moving away from you, it looks like the candle is getting dimmer and dimmer. But this is not the case. The candle is still burning at the same brightness it was even when it was close to you. What is changing is how much space the candle is lighting. This means that light is being spread over a larger area and appears to be dimmer than it is. Scientists have created a mathematical relationship to describe the brightness of an object depending on how far away it is from the viewer. Scientists also know objects in the sky that will always have the same brightness. For example, scientists know that when a certain type of star explodes it always explodes with the same level of brightness. When scientists are looking in their telescopes and find these types of stars, they automatically know how bright it should be and compare it how it appears in their telescope. Which this comparison they can calculate the distance to that star. This practice beings to fall apart once you find the last object that you know the brightness of. This then becomes the final distance you can calculate using this technique.

Figure 2 schematic of how redshift works. Depending on the movement of the object, the object will appear redder or bluer to the observer. 

The final step to take when finding the distance of the furthest objects in our universe is to find the redshift of the object. Redshift occurs when an object is moving away or towards us. In fact, you have experienced this every time you hear an ambulance. Have you ever wondered why when an ambulance is speeding towards you it sounds very loud but once it moves past you the volume decreases? The siren isn’t lowering its volume but the sound waves are just being stretched and compressed causing a change in volume. This also happens with light. When a star, for example moves away from you, the light it originally released gets stretched, so that it looks redder than it originally was. The opposite occurs when a star is moving towards you; the star appears bluer! This effect becomes more noticeable with objects that are really far away. The value that is given to redshift is related to the amount of time it takes light to travel to your eyes or telescope. By knowing how long it takes the light to reach our eye we can calculate the distance to that object because we know how fast light travels.

Figure 3.  Hubble images of quasars. In some of the images, you can see the jets of the quasar from the  gas being sucked into the black hole.

But what do all of these distance measurements have to do with time? When we are looking at the night sky, you are actually looking at the past. Light travels at incredible speeds but our universe is so large that light becomes delayed. Let’s take our sun for example. If the sun were to explode right now, it would take 8 minutes for us to notice because light takes 8 minutes to travel from the Sun to the Earth. Thus, the objects that are the furthest away from us are the ones that help us peek into the early beginnings of our universe.  The furthest objects observed by astronomers have been called Quasars. Quasars are the brightest objects in our universe and are made up of a black hole and gas. These objects are hypothesized to be created when two galaxies collide with one another and the black holes at the center of each galaxy becomes one and starts eating all the gas from the galaxies. Imagine you’re looking a tub full of water.  Everything is still until you pull the plug from the drain and the water begins to spiral into the drain. The motion of pulling the plug is identical to two galaxies colliding one another. Quasars have become an important aspect of understanding the history of our universe. Without our ladder to the universe our grasp of our universe would be unbelievably limited.